Bonding polyolefins to metal with chromium trioxide



United States Patent 3,542,605 BONDING POLYOLEFINS TO METAL WITH CHROMIUM TRIOXIDE George R. Harvey, Jr., Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed May 3, 1968, Ser. No. 726,545 Int. Cl. C23f 7/26 U.S. Cl. 1486.2 11 Claims ABSTRACT OF THE DISCLOSURE Polyolefins are bonded to metal by treating the surface of the metal with chromium trioxide, or concentrated aqueous solutions thereof, with or without a heavy mineral acid, such as sulfuric, nitric or hydrochloric acid, and in the case of aqueous solutions drying the metal surface, and in all cases then heating the metal surface and/ or the polyolefin above the melting point of the polyolefin but below the temperature of decomposition of the chromium compound, applying the polyolefin as a coating to the metal surface in the form of a plastic sheet, an extrusion in plastic to molten form, a fluidized bed of polyolefin particles, or a moving pile of polyolefin particles, and finally heating the coated metal above the decomposition temperature of the chromium trioxide.

FIELD OF THE INVENTION This invention relates to class 117, coating: processes and miscellaneous products; subclass 21, with application of fibers or particulate material, with fusion or coalescence of particulate material, and subclass 132, metal base with resinous coating. Other classes and subclasses may be more remotely involved.

DESCRIPTION OF THE PRIOR ART The closest known prior art is US. patents to: DeHart, 3,351,504 (1967); Busse et al., 3,043,716 (1962); Current et al., 2,880,109 (1959); and Gemmer, 2,974,059 (1961).

The failure to obtain a bond in Example 2, column 3, lines 1-17, of DeHart is due to heating the metal to 400 F., above the decomposition temperature of the CrO before the application of the plastic powder. The polymer face is therefore not oxidized, as it is in the present process of applying the molten polymer to the metal and then heating above the decomposition temperature of the CrO whereupon the decomposing material oxidizes the plastic surface and bonds it firmly to the metal.

' In Busse et al., FIG. IV shows spraying a metal sub- .trate with s-triazine, where as the present invention preferably applies a concentrated aqueous solution of chromium trioxide to the metal. Busse et al. in column 1, lines 35-39, state the prior art applied chromic acid to the surfaces of polyolefin articles and do not state it was ever applied, to the metal surface.

- Current et al. merely show the centrifugal method of coating the interior of a metal pipe with melting solid plastic particles. 1

Gemmer merely shows coating heated metal objects in a fluidized bed of particles, such as polyethylene Particles.

SUMMARY OF THE INVENTION The process of coating an oil and grease-free metal surface with a polyolefin, such as a polymer of a l-olefin, such as polyethylene, polypropylene or polybutylene, or copolymers of l-olefins, comprising the steps of applying a concentrated aqueous solution of chromium trioxide, preferably from 0.045 to 3 and more preferably from 0.09 to 1.5 percent by weight of chromium trioxide in water, to said metal surface, evaporating said water to deposit a layer of chromium trioxide on said surface, heating said surface and/or said polymer to a temperature above the melting point of said polymer but below the temperature of decomposition of the chromium trioxide which is about 395 F., preferably to from about 300 F. to below 395 F. and more preferably between 350 to 385 F., contacting said surface supporting the remaining portion of said chromium trioxide layer thereon with said polymer, and then heating the coated metal surface above the temperature of decomposition of the chromium trioxide, preferably to from above 395 F. to about 600 F. and more preferably between 400 to 550 F. The significance of 385 F. is that it is the melting point of the chromium trioxide, and decomposition occurs at about 395 F. and is complete before reaching 400 F.

The polymer may be contacted with the surface in the form of a sheet of polymer, an extrusion of polymer in plastic to molten form, or as a fluidized bed of solid polymer particles fluidized by any gas reasonably inert at the temperature used, such as air, nitrogen, carbon dioxide or other substantially polymer-nonoxidizing gas, or as a pile of solid polymer particles sliding and tumbling on the metal surface inside of a rotating pipe, or the like.

Any metal may be coated, such as iron, steel, aluminum, copper, zinc, brass, bronze or magnesium, and the metal surface should preferably be rendered oil and grease free by washing with an oil and grease solvent, such as acetone, chloroform, carbon tetrachloride, trichloroethylene, or the like. The remains of prior corrosion-inhibiting treatments, such as treatments with iron phosphate or zinc phosphate, and/or prior chromic acid treatments below 0.045 weight percent, have substantially no effect on the present invention. If the metal is fairly clean to start with, this degreasing step can be omitted, especially as the chromium compounds listed in the next paragraph below have some degreasing properties themselves.

'While chromium trioxide or its concentrated aqueous solution is preferred, strong mineral acids, such as sulfuric, nitric and hydrochloric acid (H I-INO and HCl) can be added to the chromium trioxide or its aqueous solution with satisfactory results.

One object of the invention is to provide an improved process for bonding polyolefins to metal.

Other objects are to bond sheets of polyolefin, extrusions of molten polyolefin, or particles of polyolefin from fluidized or moving beds to metal.

Further objects are bonding polyethylene, polypropylene or polybutylene to metal treated with concentrated chromium trioxide, sodium or potassium chromate or dichromate, or aqueous solutions thereof, with or without added mineral acid such as sulfuric, nitric or hydrochloric acid, the metal being any metal but preferably being iron, steel, zinc, brass, bronze, aluminum, copper or magnesium.

Numerous other objects will be apparent to those skilled in the art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to demonstrate the invention, several examples are given for illustrative purposes and obviously the invention as summarized above is not limited to these examples.

EXAMPLE 1 A piece of steel 4 x 6" x 0.030" had originally been treated by Hooker Chemical Corporation, Parker Rust Proof Division, 2177 E. Milwaukee Ave., Detroit, Mich., with Parkers standard Bonderite 901 (trademark) treatment, which is an iron phosphate treatment followed by a light rinse with a Cr0 solution. The steel was cut into 3 three pieces, each 1 /3 x 6" x 0.030 and labeled A, B and C, respectively. 7

Sample A was rinsed in an aqueous solution of 0.72 gram CrO /100 ml. H and allowed to air dry.

Sample B was also rinsed in a solution of 0.72 gram CrO /l00 ml. H O, but then was lightly rinsed in water and allowed to air dry.

Sample C was wiped with acetone, rinsed with seven consecutive applications of liquid trichloroethylene, rinsed with water and allowed to air dry.

Samples A, B and C were placed adjacent to each other on a square of Marlex (trademark) 5003 polyethylene resin 10" x 10" x 0.1" which had been wiped with acetone to remove any dust. This square and the supported Samples A, B and C were placed between two steel plates, all at room temperature, and this assembly was then placed between the jaws of a flat press previously heated to 430 F. Enough pressure was applied to cause slight oozing of the resin as it melted. After 5 minutes, the press was cooled by air blown on the same, said pressure being maintained until room temperature was reached. The square was removed and cut by a shearing machine into /2" wide strips cut longitudinally of and within the edges of the Samples A, B and C.

Peel strength tests were run on the Samples A, B and C with an Instron machine according to ASTM D903-49 with a backup plate at a speed of 2"/minute and with the test modified as described herein. The samples were not conditioned for seven days because that was unnecessary.

Table 1 shows that Sample A with the invention was over twice as good as Sample C without the invention but with thorough cleaning, and Sample A was also superior to Sample B to which the same 0.72 gram/ 100 ml. H O solution was applied and then washed off. This should also show conclusively that the present invention with Sample A involves reaction with the polyethylene surface and is not merely cleaning the steel surface, which is ineffective as shown by Samples B and C.

This demonstrates the beneficial effect of having a residual oxidant on the surface of the metal to make a better bond with the polyolefin.

EXAMPLE 2 The results were tabulated as follows:

TABLE 2 Peel strength l Grams CrO=l First After 7 days After 10 days 100 ml. H2O day in 140 F. H of June weather 0.0225 37 20 1/10 loose 0.045 Minimum 56 d 0.90 60 (tear 50 good 0.18 Effective rang (tear) good 0.3

8 good 0.72 60 60 good 55 1.44 Maximum 60 51 (tear) good 68 (tear) 2.88 60 56 (tear) good.-- 72 (tear) l ASTM method in 1bs./ inch.

2 The bond exceeded the strength of the material.

The steel coupons were immersed in the solution at ambient temperature and air-dried by suspending them vertically to permit drainage of the solution.

4 EXAMPLE 3 Molten polyolefin is extruded onto a coated solid metal substrate as in Step 2 of Process B of FIG. IV of US. Patent to Busse et al. 3,043,716, July 10, 1962. When the metal substrate is pretreated by coating with an aqueous solution of from 0.045 to 3 and preferably from 0.09 to 1.05 percent by weight chromium trioxide, with or without added strong mineral acid, such as sulfuric, nitric and hydrochloric acid, the metal surface is dried, the polyolefin is extruded at 300 to about 385 F. onto the metal surface to coat it, and then the coated surface is heated to between 400 and 600 F., the molten polymer adheres with a strong bond to the metal substrate.

The metal may be steel, zinc, brass, bronze, aluminum, copper or magnesium, but preferably is iron or steel.

EXAMPLE 4 Metal objects are heated and then moved about in a fluidized bed of polyolefin particles as in US patent to Ge'mmer 2,974,059 of Mar. 7, 1961. When the metal objects are pretreated as in Example 3 above and heated to 300 to 385 F. and then moved about in the fluidized bed, the polyolefin particles are melted in contact with the metal and form a coating on the objects. The coated objects are then removed and heated to from 400 to 600 R, which decomposes the chromium trioxide and oxidizes the surface of the polyolefin against the metal surface, forming'a strong bond between the metal and the polyolefin coating. The metal may be any of those named in Example 3.

EXAMPLE 5 The coating of the interior of metal cylinders as taught by Current et al. 2,880,109 of Mar. 31, 1959, is improved by pretreating and heating the metal the same as in Examples 3 and 4, which makes a satisfactory bond between the tumbling pile of polyolefin particles and the inner surface of the rotating metal pipe. The metal may be any of those named in Example 3.

While it is preferred in Examples 1, 2, 4 and 5 to preheat the metal, and in Example 3 to preheat the polyolefin, either or both may be preheated to the desired temperature between 300 and 385 F. before contact that will facilitate the contact between the metal layer and the adjacent temporarily-molten polyolefin.

While certain examples have been given for illustrative purposes, the invention obviously is not limited thereto.

Having described my invention, I claim:

1. The process of bonding polyolefin polymer to metal comprising the steps of:

contacting the surface of said metal with an oxidizing agent selected from the group consisting of chromium trioxide and concentrated aqueous solutions thereof; if the metal surface is then wet, drying the same; heating in a first heating step at least one element selected from the group consisting of said metal surface and said polyolefin polymer to about 300 F. to below the decomposition temperature of chromium trioxide of about 395 F;

contacting said polymer and said metal surface to form a bond between them; and

then heating in a second heating step to above the decomposition temperature of chromium trioxide of about 395 F. to 600 F. to form a strong bond between them.

2. The process of claim 1 in which aqueous solution of chromium trioxide is used on the metal surface which is air dried to deposit oxidizing solids thereon to contact the polymer.

3. The process of claim 1 in which the metal surface is heated in the first heating step and polymer is applied as a sheet.

4. The process of claim 1 in which the polymer is heated in the first heating step and extruded onto the metal surface.

5. The process of claim 1 in which the metal surface is heated in the first heating step and passed into a fluidized bed of particles of the polymer.

6. The process of claim 1 in which the metal surface is the interior surface of a hollow article and is heated in the first heating step, and a pile of polymer particles is tumbled inside said hollow article.

7. The process of claim 2 in which 0.045 to 3 percent by weight of the aqueous solution is chromium trioxide.

8. The process of claim 2 in which 0.09 to 1.5 percent by weight of the aqueous solution is chromium trioxide.

9. The process of claim 1 in which the metal is heated a References Cited UNITED STATES PATENTS 2,811,471 10/ 1957 Homeyer 1486.27 X 3,224,094 12/ 1965 Esemplare.

3,224,900 12/1965 Creamer et al. 117-132 X 3,351,504 11/1967 DeHart 117-21 X OTHER REFERENCES Products Finishing, March, 1964 pp. 36-42.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

20 ll721, 49, 132; 1486.27; 16l216 

